1. Field of the Invention
This invention relates to a moisture-curable hotmelt adhesive, and more particularly, to a moisture-curable polyurethane hotmelt adhesive having high green peel strength and good creep resistance.
In the field of adhesives, it is a main concern to avoid environmental pollution, fire and toxicity hazards associated with the use of adhesives such as, for example, hot-melt adhesives for bonding of various materials. Hot-melt adhesives are attractive from the viewpoint of speed of set up and ease of application. Hot-melt adhesives are widely used in the shoe industry.
However, adequate adhesive bonds are generally not provided when employing certain substrate materials commonly employed in the shoe industry using common hot-melt adhesives. One important requirement in providing satisfactory adhesive bonds is that the adhesive composition applied must have sufficient fluidity to enable it to adequately wet the substrate to which it is applied. In addition, the composition should also effect at least some penetration of fibrous substrates. Further, it is also important that the composition develops a tough, solid bond. The aforementioned requirements are particularly important in adhesively bonding shoe soles to shoe uppers where the finished product is subjected to variations in stress under different conditions of use.
Moreover, commonly known adhesives are solvent-based and create environmental, fire and toxicity hazards. Thus, a solvent-free adhesive which avoids such hazards, is easy to apply and cures to form strong bonds to a variety of substrates, for example, as in the manufacture of shoes, in an economically acceptable time, is commercially very desirable. Indeed, for many years it has been desirable to provide an adhesive applicable as a hot-melt without risk of such hazards and which cures to provide strong sole attaching bonds on a variety of substrates.
However, it has been found that a main drawback of hot-melt adhesives is their poor green peel strength. Even with the wide range of polyester materials that are commercially available, it has not previously been possible to increase the green peel strength of such adhesives without raising their viscosity to undesirable levels or lowering their cold temperature flexibility.
2. Discussion of Related Art
In the prior art there are described and claimed various methods of adhesively bonding a shoe sole to a shoe upper using selected urethane prepolymer compositions which are provided as fluid compositions and which are convertible by a chain extension treatment to a polyurethane which is tacky and capable of adhesive union when heated and so to establish substantially immediate strong adhesive union with a compatible adhesive or resin surface. The chain extension is preferably done at higher temperatures under humid conditions, e.g. in a steam cabinet or alternatively at room temperatures and relative humidities of 20 to 60%. Such methods have the advantage of employing a substantially solvent-free adhesive composition applied at a comparatively low temperature, but a significant disadvantage accompanying use of the adhesive compositions is the considerable time needed for chain extension of the prepolymer after application. The uppers which are cemented with the prepolymer have to be cured for significantly long periods of time at comparatively high temperature and comparatively high humidity in order to give required chain extension sufficient to obtain a bond of acceptable initial strength.
Attempts to improve the speed of chain extension, for example, by appropriate selection of raw materials have met with some success, but the practical requirements for tolerance of chain extension conditions and of delays prior to heat activation conditions used in a shoe factory, together with the need for producing strong bonds to a variety of substrates pose a significant problem.
One attempt to overcome said problems is disclosed in Germany Patent 26 09 226 equivalent to Great Britain 1,540,634 and Canada 1,077,352 which is directed to a process for adhesive bonding of surfaces by applying a polymeric urethane containing reactive NCO groups and segments having a comparatively low crystalline melting point to the surface of an article, and contacting the urethane with a compound supplying active hydrogen atoms which react with the NCO groups to cause a chain extension therein.
Another adhesive composition is disclosed in Great Britain 2, 137,638 wherein a moisture-curable adhesive comprises polyurethane prepolymers having NCO groups available for reaction, and a monofunctional reactant present in amounts sufficient to react with up to 40 mole percent of the isocyanate groups of the prepolymer. The polyurethanes are preferably based on polyesters from aliphatic dicarboxylic acids and diols together with aromatic diisocyanates. The monofunctional reactant is preferably an aliphatic primary alcohol and is said to influence the extended ability of a chain extended layer of the adhesive to be activated by heat after comparatively long periods and provide good bond strengths.
A further method of bonding a sole to a shoe is disclosed in European Patent Specification 223,562 by applying a moisture-curable, solvent-free melt adhesive to the sole and/or shoe upper, subjecting the adhesive coating to a hot-cure step with airborne moisture, and pressing the sole and shoe upper together with the adhesive between them immediately after the hot-cure step, preferably in less than 90 seconds after leaving the hot-cure.
However, the prior art methods and adhesive compositions suffer from numerous disadvantages. That is, characteristics desirable for an adhesive composition for application in at least substantially solvent-free form for sole attaching purposes include the following properties. The adhesive composition should be capable of application in fluid form at a temperature which is sufficiently low that the shoe making materials used are not adversely affected. The adhesive composition should have at such temperatures good heat resistance, viscosity and flow characteristics which enable it to properly wet surfaces to which it is applied and preferably also to penetrate fibrous materials, for example, leather, whereby to establish good initial adhesive bonds to the materials. Coatings of the adhesive present on the materials should have good creep resistance, and harden adequately within a few minutes to provide at least a non-tacky surface and preferably a solid form-retaining non-tacky coating on the material to the extent that soles and shoe uppers coated with the adhesive composition may be handled and stored when cooled without sticking together and without significantly disturbing the applied layers of adhesive composition. It is also desirable that the adhesive union achieved between the sole and shoe upper should initially be sufficiently strong to avoid springing of the sole edges from the shoe upper during handling and storage immediately after sole pressing, i.e., have good green peel strength, and should be or become adequately cured to withstand normal conditions of use of the shoe as epitomized by ability to withstand a peel test at 5 kg per cm width carried out 72 hours after sole pressing.
In addition, when using an adhesive composition comprising a moisture curable polyurethane hot-melt prepolymer, it is desirable that coatings of the adhesive composition on the shoe quickly cure a short time after application to bring about conversion of the prepolymer to a tough, solid condition.
Further, when using an adhesive composition comprising a moisture curable polyurethane hot-melt prepolymer, it is desirable that coatings of the adhesive composition on the shoe should be susceptible to the effect of moisture in the atmosphere under normal, ambient conditions of storage of the soles, i.e. approximately 20.degree.+/-5.degree. C. and relative humidity of not less than approximately 10% to bring about conversion of the prepolymer to a tough, solid, cured condition. It is convenient if this condition is at least partially achieved during storage overnight, and it is highly desirable that the conversion of the prepolymer of an adhesive composition coating on the shoe progresses during storage in such a way that the coating becomes fully cured in a period of up to about three days after application.
In addition, the cured adhesive composition should have good flexibility at low temperatures, and provide a strong bond to a variety of materials, for example, as employed in the shoe industry.